oh sh*t this cant be good! need help fast

ok well today me and my brother hooked up his system and everyting was fine so start. this wasnt my first install either. but today after he was out ridin around he stopped to talk to somone and show him his new ****. the fuse somhow blew but it still looks fine i checked it witha tester and its junk. but after he put a new one back in it was fine and started bumpin again. after awhile he stopped somwhere and shut his car off. when he was goin to leave his car kept on stallin and ****. it would run for a few seconds and die. also his power windows trunk interior lights and all that would not come on. i was wonderin if any of you had a clue on what the hell i might have messedd up and if it would be possible for me to fix. his car is a 1990 caddy fleetwood. all he was runin was like 250-300 rms. his amp was bridged at 4ohms. thanks for any help on this becuase it kinda lost me.

sounds like his alt. is not providing enough power to run his car...have him try driving w/ his sterio off and see if it still happens....if it runs fine then he might need to get his alt. rewound or a new high output alt....othre than that i cant imagine what it would be...good luck